Datasheet

178
ATmega8A [DATASHEET]
8159E–AVR–02/2013
21.8.4 TWDR – TWI Data Register
In Transmit mode, TWDR contains the next byte to be transmitted. In Receive mode, the TWDR contains the last
byte received. It is writable while the TWI is not in the process of shifting a byte. This occurs when the TWI Interrupt
Flag (TWINT) is set by hardware. Note that the Data Register cannot be initialized by the user before the first inter-
rupt occurs. The data in TWDR remains stable as long as TWINT is set. While data is shifted out, data on the bus
is simultaneously shifted in. TWDR always contains the last byte present on the bus, except after a wake up from a
sleep mode by the TWI interrupt. In this case, the contents of TWDR is undefined. In the case of a lost bus arbitra-
tion, no data is lost in the transition from Master to Slave. Handling of the ACK bit is controlled automatically by the
TWI logic, the CPU cannot access the ACK bit directly.
• Bits 7:0 – TWD: TWI Data Register
These eight bits constitute the next data byte to be transmitted, or the latest data byte received on the Two-wire
Serial Bus.
21.8.5 TWAR – TWI (Slave) Address Register
The TWAR should be loaded with the 7-bit Slave address (in the seven most significant bits of TWAR) to which the
TWI will respond when programmed as a Slave Transmitter or Receiver, and not needed in the Master modes. In
multimaster systems, TWAR must be set in masters which can be addressed as Slaves by other Masters.
The LSB of TWAR is used to enable recognition of the general call address (0x00). There is an associated address
comparator that looks for the slave address (or general call address if enabled) in the received serial address. If a
match is found, an interrupt request is generated.
• Bits 7:1 – TWA: TWI (Slave) Address Register
These seven bits constitute the slave address of the TWI unit.
• Bit 0 – TWGCE: TWI General Call Recognition Enable Bit
If set, this bit enables the recognition of a General Call given over the Two-wire Serial Bus.
Bit 76543210
TWD7 TWD6 TWD5 TWD4 TWD3 TWD2 TWD1 TWD0 TWDR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value11111111
Bit 76543210
TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE TWAR
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value11111110